As announced in the thread on bugs and issues in 4.1.0, I 'd like to float some ideas on how to expand and integrate perception in the game more fully, now that it has been revived.
Most of these ideas are suggestions for future development rather than issues of 4.1.0, so I decided to put them in a separate thread.
I split the theme into 3 parts, to reduce the chance on a bloated stomach...

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Two monsters to turn cavers into cadavers,
But only when together,
With the small one in front.

Perception has long been a 2nd-rate skill with little or no impact. That has now changed. In 4.1.0 it is the primary skill to help you spot hidden traps. Which is nice, imo. But there are a lot more game elements where it could/should play a role:

(I am not suggesting to stuff all of these into vanilla; just some.)

Perception (skill) can affect how good you are in ...

spotting doors from more than 1 tile away (but not from huge distances)

spotting mimics and other hidden monsters (trappers, creeping coins, ...). Preferably on time; before you step next to them and are stung with paralyzing venom, and/or bitten for other nasty effects (preferably worse than just poison with those that move slowly or not at all!).
When this is to apply to mimics too, we must prevent mimics from casting a spell at the very moment the player comes into LoS, to give @s that are sufficiently perceptive and stealthy a decent chance to spot the mimic and prevent havoc. At present that chance is zero, because mimics are always vigilant, and start picking up noises when the player is far away (hearing=25). Suggestion: nerf that to something like hearing=12, sleepiness=2.

sensing weird-minded creatures with esp (eg, perception/2 % chance)

hearing doors burst open, wolves howl, rock eaters grind, ...

getting a level feeling on treasure (if you step on a "feeling-tile", you get a chance of perception% to get a level feeling.)

spotting buried treasure (as a dwarf)

pseudo-id (ah, removed? really? what a pity )

Add new ways of "sensing" the vicinity of monsters (without knowing their exact positions). To help warriors a bit, and compensate others for nerfing detection (as below):
Perception (skill) can affect how good you are in ...

how good you are in recognizing monsters that appear at distances near the end of your visual range (eg 100% at 160 ft, perception% at 200 ft; linear inbetween).
An unrecognized D would be "an ancient dragon" if you 'l'ook at it from afar. Only if it gets close enough for you to recognize it (or possibly if you cast monster detection), you'd see that it is not a dracolich but 'just' an ancient black dragon.

how often you overlook monsters that are (visually) stealthy. Chameleons are not there yet, so think of ninjas. The rule would be similar to the previous item, but with the 100% point much closer to @ (say, 40 ft away). So that, with so-so perception, @ in a large room may suddenly discover that he is surrounded by ninjas. Ouch.

how often (depending on monster distance vs perception skill) you overlook stealthy monsters in magical detection.

To embed the perception skill in the game more fully, the impact of player states on perception must be decided on. Below are some suggestions.

Player states that should probably affect perception:
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confusion......................(a big debuff)

hallucination..................(a big debuff)

starvation.....................(a ... debuff?)

poisoning......................(a ... debuff?)

stunning.......................(a modest debuff)

berserkness...................(a modest debuff)

fear.............................(a modest buff? or a modest debuff?)
I could imagine that perception is slightly better than normal if you are nervous, but worse than normal when you are panicking.

terror...........................(a big debuff)

blindness........................(??)

If perception involves other senses besides sight, blindness should not reduce perception per se. But of course, a blind @ should not be capable of spotting a trap (except when hitting it). So blindness should probably be checked separately whenever perception plays a role.

see_invisible.....................(a big debuff)

Thinking of the "flowing forms" that Frodo sees (in the LotR film) when he puts on the ring to escape Boromir, I 'd say that wearing a ring of see_inv should incur a significant debuff of perception. That would also mean that @ can recognize such a ring immediately when trying one on, even without any invisible creatures present.
Some items (HA, helm of seeing, ...) should probably get a perception boost to compensate for the debuff (or - for the helm - more than compensate), but it seems appropriate to let see_inv always hurt normal perception.

You mentioned ESP, but I happened to be thinking in even more radical terms:

Change ESP to only give to you slight hints that there are monsters ahead, but don't show the "letters". (This is from Sil. That's one of the few mechanics that I enjoyed from Sil.). Let Perception skill add a little bit to that so that you can occasionally see the "letter"!

(I should really try this out in my own T2-ah variant, but I still have a huge amount of clean up to do before the code is amenable to experimentation like this.)

It has been noted (by myself and others) that certain classes in 4.1.0 have such low perception skill that running into traps again and again is a bother.
There is a pretty wide gap indeed between the extremes of a kobold rogue and a troll warrior. A kobold rogue's "bare" perception skill soars from 30 at clevel 0 to 100 at clevel 43, whereas a troll warrior's bare skill starts at 1 and ends at 61 at clevel 50. So if you like playing troll warriors (or mages or paladins), you may feel that perception boosters are important.
Also, the role of perception in the game is currently limited (to spotting traps), but it seems natural to have perception impact nearly everything a character does. With such a key skill, it is important to ensure that characters who happen to have poor perception can find relief in perception boosters.

The only boosters in 4.1.0 are wearable items (ring, amulet, light source, headgear, ...). That's a lot in comparison with disarming skill, for which there are no boosters at all (AFAIK). But it's pretty meagre in comparison with the wide spectrum of boosters for melee/archery.
Next to weapons, these include armor and jewelry, and several potions, mushrooms and scrolls. And on top of that, there are indirect boosters: monster state attacks, glyphs, ac boosts, and speed boosts. These help melee, archery and spellcasting, but are irrelevant for perception.
Others (Pete Mack and Derakon) have suggested that ambient light can boost perception. In this post, I'll discuss skill boosters only, so that discovering hidden traps is always a matter of pitting player perception skill against trap stealth (aka visibility). Here, the stealth of a (specific) trap is fixed; buffs and de-buffs work on the player skill only.

I think that the 3 blessing prayers (and the scrolls) are natural candidates.
Next to offense and defense, let them buff perception and disarming. And maybe even saving throw.
It seems fitting that "blessing" would involve more than just fighting buffs.

Some mushroom could provide a bigger buff to perception (and disarming), but slow you down.
A nice dilemma when you want to disarm a summoning trap...

Alternatively, or in addition to a mushroom, I was thinking of 'S', but that may be problematic in the current framework with traps at semi-random chokepoints.
(see below, in the last paragraph)

I also considered de-buffing perception when running.
It seems natural: you notice more of the environment when you take one step at a time than when you run. Except, of course, that 'running' is not really running, but just auto-exploring. You tell your character to explore in some direction, and "not report back to you until it finds something interesting". There's no haste involved here.
OTOH, it does feel right to me: if I press left-left-left-... it feels like being watchful; if I press Shift-left or . left, and see @ tear across the screen, it doesn't.
But that may well be different for the somewhat more hasty players in our community, for who "taking one step at a time" probably means that they are hammering on that cursor key like there's no tomorrow, instead of pressing it gently and thoughtfully, as i do.

Another player dislikes key hammering too, and has requested a hi-speed auto-hammer.
[Pfui. No wonder that Treebeard mistook hasty hobbits for young orcs...]
Anyway, in the current framework (with traps at semi-random chokepoints), there can be a trap anywhere in a corridor. In that context, nerfing perception while exploring comes down to a ban on exploring for "optimizing" players. So such a debuff is unacceptable.

The same line of reasoning applies, more or less, to a buff of perception from 'S' (with the old semantics of better perception at the cost of halving speed).
More or less, but not quite.
For "having to press S" before you start exploring is much less of a bother than being unable to explore. You can even include it as a fixed element in an explore macro. And, in contrast with exploration, entering 'S'earch mode would incur a real change of perceptiveness. Also, provided that you do NOT let 'S' boost stealth as well (as I originally intended), there's actually an interesting tradeoff: Do you try to minimize the risk that @ runs into a trap, or do you try to minimize the chance of monster wakeup? In the first case, you hit 'S' before exploring; in the second case you don't.

Last edited by dreembeard; October 7, 2017 at 00:54.
Reason: a forgotten smiley

You mentioned ESP, but I happened to be thinking in even more radical terms:

Change ESP to only give to you slight hints that there are monsters ahead, but don't show the "letters". (This is from Sil. That's one of the few mechanics that I enjoyed from Sil.). Let Perception skill add a little bit to that so that you can occasionally see the "letter"!

(I should really try this out in my own T2-ah variant, but I still have a huge amount of clean up to do before the code is amenable to experimentation like this.)

Sounds interesting. But that is a really big difference with current vanilla! Now, esp not only reveals the race family (say 'T'), but also the exact race: whether it's a forest troll, or a stone troll, or even some troll you've never seen in the flesh yet.